The present invention relates to assisted global positioning system (GPS) devices, and in particular, to GPS devices that include wireless communications capability.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Global positioning system (GPS) receivers can provide location information by communicating with satellites that orbit the earth. Such GPS information may provide positioning accuracies that are superior to alternative technologies such as cellular's cell-ID. A GPS receiver typically locates four or more satellites and calculates the distance to each satellite by timing a radio signal from each satellite to the receiver. In order to use this timing information the receiver, has to know the location of the satellites. Since the satellites travel in known orbit paths, the GPS receiver can receive and store the ephemeris and/or almanac that tells the receiver the location of the satellites at various times. Slight variations on the positions of the satellites caused by planetary gravitational pulls are also monitored by the Department of Defense and transmitted by the satellites themselves as part of their signals to the receivers.
A GPS receiver uses the distance/timing information and the location of the satellites to calculate its own position. The distance from each satellite limits the location of the GPS receiver to a sphere centered on the satellite location with a radius equal to the distance to the satellite. The intersection of the four spheres (one for each satellite) provides the location of the GPS receiver.
In order for the GPS receiver to measure the travel time between the satellite signal and the receiver, the satellite transmits a pseudo-random code pattern. The receiver also starts playing the same pattern. After the satellite signal reaches the receiver, it exhibits a delay lag compared to the receiver pattern. This delay represents the signal travel time. Both the satellite and the receiver must use the same clock time in order to measure this delay. The satellites use expensive atomic clocks, whereas the receivers use inexpensive quartz clocks that are recalibrated at frequent time intervals. The receiver looks at signals from four or more different satellites. It then draws the corresponding four or more spheres. If there is timing inaccuracy for the receiver's clock, the spheres will not intersect at a single point since they will all be offset by the same time error. The receiver can then adjust its clock until the spheres intersect at a single point, thereby synchronizing with the atomic clocks of the satellites.
GPS receivers may be categorized into two types, autonomous GPS receivers and assisted GPS (A-GPS) receivers. These types are detailed below.
Autonomous GPS receivers receive information from orbiting satellites and perform the calculations themselves in order to locate their position. The information they download from the satellites includes satellite ID, almanac, clock corrections and ephemeris. They use this information along with their measured pseudorange data to calculate position and velocity. Autonomous GPS systems, however, have relatively low sensitivity and cannot obtain positions in low signal environments. They also have large initial satellite signal acquisition times, since they need to decode the data received from the satellites.
Assisted GPS systems resolve the sensitivity and acquisition time limitations of autonomous GPS receivers by using assistance data from a networked and typically AC powered assistance server. The mobile device's assisted GPS receiver measures pseudoranges and Dopplers, and obtains assistance data from the assistance server. The assistance data from the server can include satellite ID, almanac, clock correction, ephemeris, approximate time, and approximate location.
Assisted GPS systems are categorized into two classes: mobile-based A-GPS and mobile A-GPS (i.e. mobile-assisted A-GPS). A mobile-based A-GPS receiver measures pseudoranges and Dopplers, receives the assistance data from the assistance server, and performs the position calculations itself. A mobile A-GPS receiver, on the other hand, uses the more powerful assistance server to perform some of the calculations for determining its position, thereby saving the mobile device's limited battery and CPU resources. For example, the mobile A-GPS receiver can calculate the distance to the satellites from the satellite signals and transmit these to a cell base station tower, which in turn forwards them to the assistance server that performs the position calculations. The assistance server can then send the calculated position information back to the mobile A-GPS receiver (or in the case of emergencies, to a 911 dispatcher).
The accuracy of A-GPS is limited by variations in the speed of the radio waves of the satellites as they travel through the earth's atmosphere. Differential GPS (DGPS) reduces these errors and all other common errors (e.g., satellite clock error) by using a stationary receiver with known coordinates. This receiver compares its computed position with its known position to calculate inaccuracies caused by atmospheric effects and other common errors in that area. It then broadcasts correction data to nearby DGPS receivers, making them more accurate than standard GPS receivers.
One problem in many existing GPS systems results from multipath signal interference. Multipath signal interference occurs when the GPS signals from the satellite are reflected by objects around the GPS receiver, such as buildings. In areas with many buildings, such as city centers, multipath signal interference results in significant position errors. There is a need for GPS systems that reduce multipath signal interference.
Similar problems include shadowing and fading. Shadowing can be caused by a large obstruction that is along the main signal path between the transmitter and the receiver. Shadowing can produce amplitude and phase changes on a carrier modulated transmitted signal. Fading also describes the distortion that a carrier-modulated signal undergoes as it travels from the transmitter to the receiver. Multipath propagation is one of the main causes of fading since it results in the receiver receiving and superimposing multiple copies of the transmitted signal, each traveling along different paths and experiencing different phase shifts, attenuations and delays. There is therefore a need for GPS systems that reduce shadowing and fading.
Thus, there is a need for improved GPS systems. The present invention solves these and other problems by providing methods and systems for assisted GPS devices.